1. Field of the Invention
The present invention relates to a process for efficiently producing carboxylic acid esters from carboxylic acid amides and formic acid esters or alternatively from carboxylic acid amides, alcohols and carbon monoxide.
2. Description of Related Arts
Carboxylic acid esters are industrially important compounds. These carboxylic acid esters can be produced from carboxylic acid amides: for example, methyl acetate is produced from acetic acid amide; methyl methacrylate from methacrylic acid amide; methyl acrylate from acrylic acid amide; or methyl .alpha.-hydroxyisobutyrate from .alpha.-hydroxyisobutyric acid amide. For producing carboxylic acid esters from carboxylic acid amides, a method of reacting carboxylic acid amides with alcohols in the presence of sulfuric acid is well known. This method is widely employed for industrial production of methyl methacrylate.
This method, however, has disadvantages in that a large amount of acidic ammonium sulfate results as a by-product, leading to a marked increase in production costs owing to its disposal, and expensive corrosion-resistant apparatus is also required.
In order to overcome the above problems, a method of producing carboxylic acid esters by reacting carboxylic acid amides with alcohols without the use of sulfuric acid has been proposed. In Japanese Patent Application Laid-Open Nos. 3015/1977, 141216/1978 and 144524/1978, for example, the reaction is carried out in a liquid phase in the presence of e.g. metal salts or metal alcoholates. On the other hand, in Japanese Patent Application Laid-Open Nos. 67534/1982 and 49338/1983, the reaction is carried out in a gas phase in the presence of a solid acid catalyst.
These methods, however, have disadvantages in that the yield of the desired carboxylic acid ester or its selectivity is low and thus are not satisfactory for commercial practice thereof. Moreover, in commercial practice, they suffer from problems in that: (1) the reaction should be carried out at high temperatures, (2) in case of the liquid phase reaction, high pressure is needed, (3) a large amount of ammonia is produced during the reaction and thus its recovery and separation is needed, and (4) the reaction of the ammonia with carboxylic acids as by-product leads to the formation of ammonium salts thereof.
A method of producing carboxylic acid esters and formamide by reacting carboxylic acid amides with formic acid esters, or alcohols and carbon monoxide has been developed, as described in Japanese Patent Application Laid-Open Nos. 55444/1983 and 78937/1985.
In Japanese Patent Application Laid-Open No. 55444/1983, main catalysts comprising metal salts of organic or inorganic acids, or metal chelate compounds, and an accelerator comprising nitrogen or phosphorus-containing organic compounds are used in combination. In one of the examples, the reaction was carried out at 185 to 250.degree. C. for 2 to 4.5 hours, and the desired carboxylic acid ester was obtained in a yield of 16.3 to 78.9%. This yield, however, cannot be said to be sufficiently high, and moreover the catalyst system is expensive.
In Japanese Patent Application Laid-Open No. 78937/1985, a catalyst system consisting of the combination of amidine or tertiary amine and metal carbonyl is used, and the reaction is carried out under pressure of carbon monoxide. However, although the selectivity of the carboxylic acid ester is relatively high, this method has disadvantages in that high pressure is needed for the reaction, highly toxic metal carbonyl is used, and the catalyst system is expensive.